The Mayor and the Geotechnical Engineer – Challenges Obtaining Approval for Use of Alternative Fill Materials to Accommodate Sea Level Rise in the City of San Francisco

Mission Rock is a new waterfront neighborhood in San Francisco. Recently completed, Phase 1 consists of two, 240-foot-tall residential high-rise buildings, a 120-foot-tall and a 210-foot-tall commercial building, a waterfront park, and new public streets and infrastructure. The site is on land owned by the Port of San Francisco.  The streets and infrastructure would be maintained by the City and County of San Francisco (CCSF) Public Works and the San Francisco Public Utilities Commission (SFPUC). Approving and permitting agencies consisted of the Port of San Francisco, CCSF Public Works, and the Department of Building Inspection.  The site is located along the San Francisco Bay and was originally below water in a shallow bay, which was reclaimed between 1880 and early 1900’s. The future sea level is predicted to rise as much as 66 inches by the year 2100, which would submerge the site during a high tide. Raising grades using conventional soil fill to account for future seal level rise was not feasible due to the presence of compressible Bay Mud; the weight from compacted soil would cause as much as 2 feet of consolidation settlement. A number of options were considered to allow for raising grades without causing settlement. Ultimately, the team selected to offset the new loads by overexcavating existing fill and raising grades using lightweight cellular concrete (LCC); however, LCC was not approved for use as fill by the City of San Francisco or the Port of San Francisco. The various stakeholders were uncomfortable with any approach that did not include support of all utilities and roadways in conventional soil.  Their concerns included strength, resistance to uplift, performance during an earthquake, excavatability, long-term changes in weight due to saturation, and availability and patent concerns for future replacement. In order to gain approval and prove out the proposed methodology toward constructing a sustainable and resilient new neighborhood, the project owner and design team embarked on a lengthy process, including multiple peer reviews and meetings with the Director of Public Works, the City Engineer, the Chief Harbor Engineer from the Port of San Francisco, and even the Mayor. This paper will present this process, the concerns of the stakeholders and how they were addressed, towards obtaining approval of this non-standard fill material.